This relates to commonly assigned U.S. Ser. No. 943,794 filed Dec. 19, 1986, now U.S. Pat. No. 4,728,680 and the disclosure of said application incorporated herein by reference. This invention pertains to emulsion coatings treated by cation exchange where the emulsion polymers contain hydroxyl or carboxyl groups but are free of amine groups. The stable reactive coating is based on ion-exchange of a water dispersed mixture of glycoluril resin and acrylic emulsion polymer containing carboxyl and/or hydroxyl groups but free of amine groups, where certain cation salts are added to increase the pH of the coating without adversely affecting the thermosetting cure of the coating.
Prior to this invention, glycoluril cured thermosetting emulsions were produced from non-ionic and acidic surfactants and generally required high levels of such surfactants. Anionic salt surfactants cause a detriment to the curing reaction while pigmentation of such glycoluril and emulsion mixtures cause undesirable flocculation during the pigment grind step and subsequent let-down to produce pigmented mixtures.
Prior art U.S. Pat. Nos. 4,442,257 and 4,444,941 teach the use of certain acrylic latexes crosslinked with either tetramethylol glycoluril (TMGU) or dihydroxy dimethylol ethylene urea (DDEU) for low-temperature curing coatings. Rapid cure is achieved with these products by adding from 1.5% to 3.5% by weight of a 40% solution of paratoluene sulfonic acid where a package pH of about 1.0 to 2.0 results. Stability of these catalyzed products ordinarily is limited to 1-5 days at room temperature, and much less at elevated temperatures. Hence, the acid catalyst must be added at the time of use. Cured films of such polymeric mixtures often exhibit certain water sensitivity due to residual catalyst in the cured films. Without the acid catalysts, however, the coating has no solvent or water resistance unless baked at extremely high temperatures.
Abbey (U.S. Pat. No. 4,525,260) discloses a cationic latex for cathodic electrocoating and specifically includes amine monomers. The reference latex is based on acrylic monomers copolymerized with amine monomers, such as N,N-dimethyl-2-aminoethyl methacrylate, and catalyzed with an azo catalyst. Abbey subsequently treats the cationic latex with an ion exchange resin but to specifically remove amino monomers and amino initiator fragments. Hence, the reference is specifically directed to removing amino fragments introduced in the emulsion process. As noted below, amine latexes are not operative in this invention in that amine groups block coreactivity with a glycoluril cross-linker and inhibit the cure.
In commonly assigned U.S. Ser. No. 943,794 filed Dec. 12, 1986, low temperature cure emulsion polymers particularly useful in paint coatings are disclosed based on a thermosetting polymeric binder composition comprising a functional addition polymer containing functional hydroxyl or carboxyl groups, but free of amine groups, and a glycoluril derivative adapted to be coreactive with the functional polymer. The functional polymer and preferably the glycoluril derivative are treated with an ion exchange process step to remove undesirable cations. The resulting resin provides a highly desirable low temperature cure paint coating adapted to thermoset without the addition of undesirable acid catalysts. However, it has been found that cation exchanged polymeric mixtures based on functional emulsion polymers adapted to be coreactive with glycoluril derivatives and pigmented with considerable amounts of conventional commercial titanium dioxide exhibit an upward pH drift of as much as one pH unit within a few days after cation exchange of the polymeric binder and/or the paint coating, which does not seem to materially effect the stability or low heat cures, but nevertheless limits the commerical utility of process to darker color paints.
It now has been found that cation-exchanged polymeric compositions, wherein the great majority of soluble cations have been replaced by protons, the composition comprising functional emulsion polymers and coreactive glycoluril used in paint coatings can be substantially improved by the addition of additive amounts of certain cation salts to adjust the pH without loss of cure and thermosetting properties. The use of certain cation salts to adjust the pH upwardly provides considerable flexibility to paint formulations including versatility with respect to light colored pigmented paint coatings. The addition of various mono- and polyvalent cation salts to cation-exchanged reactive latex/glycoluril blends cause an increase in the pH of the blend with no detrimental effect on cure properties. By using this technique, the pH of cation-exchanged glycoluril coatings may be adjusted to reduce the possibility of hydrolysis or degradation of certain paint components and/or simplify pigmentation, for example, without inhibiting the crosslinking reaction. A coating pH above 2 and preferably above 3 effectively avoid corrosion problems when the coating is applied to steel substrates. These and other advantages will become more apparent by referring to the detailed description and illustrative examples.